I can cruise around in my E220 CDI all day without any noticeable exhaust smoke. I don't even see any when the engine is cold-started. Very occasionally, I will put my foot down and then I can see a diesel smoke cloud in my rear view mirror. However, if I repeat the manoeuvre then there is no visible smoke. It's like there is some build-up which is cleared when I accelerate hard. The engine's done almost 70K miles. Is this normal or does it sound like a problem? The car's going in next week for a couple of warranty issues, so should I mention it?